PURPOSE: To keep the high precision of the frequency and also to simplify the hardware constitution by deviating the frequency of a reference frequency oscillator based on the data given from a controller and monitoring the varied frequency value.

CONSTITUTION: The recognized data are sent to a data holder 8 from a controller 7 which recognizes a deviating extent Δf so that the frequency of a reference frequency oscillator 1 is deviated by Δf. The data outputted from the holder 8 are converted into the analog voltage by a D/A converter 9 and inputted to the frequency control terminal of the oscillator 1 via a higher harmonic component eliminating LPF 10. Meanwhile a loop circuit is formed to monitor a process where the oscillator 1 is correctly deviated by Δf under the control of a controller 7 and transmits an error to the controller 7 against the Δf to converge the error during the monitoring process. Then the controller 7 monitors the difference frequency data to check whether the desired difference frequency is secured or not. If not, the data given to the holder 8 are changed so that the output frequency of the oscillator 1 is set at a normal level.